Multimode cardiac pacer



March 1969 J. w. KELLER, JR

MULTIMQDE CARDIAC PACER Filed May 6, 1966 w JEQL Us INVENTOR. JOHN WALTER KELLER,JR.

ATTORNEYS United States Patent Office 3,433,228 Patented Mar. 18, 1969 2 Claims ABSTRACT OF THE DISCLOSURE A cardiac pacer which includes a pulse generating source and a ventricular electrode for applying the pulses as stimuli to the heart also includes means for sensing a heart potential by which the pulse generating circuitry will operate in response to the sensed heart potentials. The pulse generating source comprises a free running multivibrator with an R-C cross coupling path from the collector of each transistor to the base of the other transistor. Incoming signals corresponding to a sensed heart potential are connected to a diode which, when not reversely biased, passes them to the base of one of the transistors to recycle the multivibrator. A capacitor and a charging and discharging circuit maintain the diode reversely biased for a simulated refractory delay period following the delivery of each heart stimulating pulse and/ or sensed heart potential pulse. The means for sensing a heart potential may include both an atrial electrode and the ventricular electrode with switching means to select either for controlling the pacer.

This invention deals with cardiac pacers. Among its objects are to simplify the construction of an electronic cardiac pacer, to accept a Wider range of natural physiological signals from the heart and take appropriate action, to give the attending physician a wider range of choice of modes of operation, and to ensure against failure of operation of the pacer, which might otherwise result from extraneous disturbing influences.

In the normal heart the left auricle accepts oxygenated blood from the lungs and delivers it to the left ventricle which pumps it through the body tissues. At the same time the right auricle accepts de-oxygenated blood from the veins and delivers it to the right ventricle which pumps it through the lungs where the products of the body metabolism, notably carbon dioxide, are removed and oxygen is absorbed. Thus each side of the normal heart operates as a two-stage pump, the two auricles which together constitute the atrium contracting simultaneously and the two ventricles contracting simultaneously.

Contraction of the atrium is accompanied and slightly preceded by a transient electrochemical potential known in medical terminology as the P Wave. After an interval known as the AV (atrioventricular) delay of the order of 5 second another electrical potential known as the QRS complex normally appears in the ventricle and, in the normal heart, this initiates the contraction of the ventricle. Following each contraction of the ventricle it remains inert and insensitive to electrical influences throughout a period known as the refractory period which, in the human heart endures for about one-quarter second. Following immediately after this refractory period is a vulnerable period of the heart of a few milliseconds duration in which, if an electrical stimulus is applied to the heart, though it does not respond by contraction, it may be damaged.

In the specification of a copending application, Ser. No. 283,271, filed May 27, 1963, now Patent 3,253,596, granted May 31, 1966, entitled Cardiac Pacer, apparatus is described which, implanted in a patients body, responds to an atrial P-Wave, when it exists, and, after a simulated AV delay, delivers a stimulus to the ventricle, causing it to contract. This action replaces the normal physiologic transmission from the atrium to the ventricle, when such normal transmission has failed. The artificial stimulus is generated by a multivibrator comprising two transistors of like conductivity types having cross couplings from the collector of each to the base of other such that, in the absence of a signal picked up from the atrium, it runs at a pre-set rate, illustratively 70 pulses per minute. This multivibrator is characterized by an ON time when one of the transistors conducts and an OFF time when the other transistor conducts. Its full period is the sum of the ON time and the OFF time, and one or other of the transistors conducts throughout the full period. During the OFF time the multivibrator can accept a signal, if present, from the AV delay simulator and this acts to switch the multivibrator into its ON state, perhaps earlier than if it were left to itself. During the ON time the multivibrator is insensitive to such a signal. Thus the ON time is the electronic counterpart of the refrectory period of the heart. The ON time is determined solely by the magnitude of the circuit elements. The OFF time is, in the absence of a signal from the atrium, similarly determined by other circuit elements but can be shortened when an atrial signal reaches the multivibrator through the AV delay simulator. Thus, in the absence of atrial signals the pulsing takes place at a pre-set fixed rate while in the presence of atrial signals it takes place in synchronism with them, albeit retarded by the AV delay.

To prevent the application to the ventricle of a stimulus throughout the duration of the ON time of the multivibrator, it is followed by a regenerative amplifier which, in response to the switching of the multivibrator from its OFF state to its ON state, delivers a brief pulse, of a few milliseconds duration. This is passed through a direct current blocking capacitor to an electrode which is surgically embedded in the heart muscle.

In some cases of heart block the P-Wave and the auricular contraction disappear or their rhythm becomes abnormal while QRS complexes, to which the ventricle responds by contracting continue, albeit at rates too low to sustain life. Consequently, while it is desirable to cause the pacer to depart from its pre-set rate in response to the P-Wave, when present and normal, it is equally important, when the P-Wave fails, to allow for natural response to a QRS complex when it exists, and to be at all times prepared to deliver an artificial stimulus when the QRS complex does not exist or does not appear when the body needs it.

In accordance with the present invention this is accomplished by the inclusion of a switch which, in one position accepts the P-Wave from the atrium as in the specification referred to above, and in the other position accepts only QRS complexes from the ventricle. The electrode which senses the QRS complex may be the same one which has already been imbedded in the heart muscle and by which the contraction-initiating stimulus is delivered to it. The potential thus picked up from the heart; i.e., P-Wave or QRS complex, is first amplified, then delayed by the AV delay simulator and thereupon applied to a multivibrator which, in the absence of sensed heart potentials, delivers contraction-promoting stimuli to the ventricle at a pre-set rate, e.g., 7O stimuli per minute. The potential-thus sensed and delayed acts to switch the multivibrator from its OFF state to its ON state in which it delivers a stimulating pulse to the heart. When the sensed potential is that of a QRS complex then, the heart muscle having just undergone a contraction in the normal physiological fashion in response to its own QRS complex, the artificial stimulus falls within the refractory period of the heart and is thus ineffective to cause a premature contraction. If, to the contrary, an abnormally long time has elapsed since the sensing of the last QRS complex, the multivibrator delivers an artificial stimulus to the ventricle which responds by contracting. Every such stimulated contraction is accompanied by a potential change in the heart which is known, in medicine, as a QRS complex. To avoid confusion the term QRS complex is employed in this specification to denote only a natural physiologic signal in contrast to one that is artificially stimulated.

Our modern world is replete with sources of electrical energy. A person who stands in the path of a radio beam, in the close vicinity of a high tension power line or the like or who seizes a household appliance to which power is being applied and of which the exterior may be imperfectly grounded is subject to the induction, within his body, of electrical fields of significant magnitudes. While these do not interfere with normal bodily activity, they may well interfere with the operation of a sensitive electronic pacer, whether implanted in the patients body or not. Such interference, when it exists, manifests itself first as a change in the potential of the heart muscle, and this, when sensed, may initiate the generation of an output pulse from the pacer and a consequent stimulus to the heart. However, even in the presence of strong interference the apparatus restricts the elicited stimuli to tolerable rates, e.g., 150 per minute or fewer, well below the rates of the interfering signals.

The stimulus required by the ventricle to initiate its contraction is very brief: its duration may be as short as a millisecond or so. This is in sharp contrast to the much longer interval (0.85 second for a pulse rate of 70 beats per minute) which elapses between successive stimuli. In the present apparatus, advantage is taken of this relation to combine the functions of the multivibrator and the regenerative amplifier of the specification above referred to by employing a pulse source of very short duty cycle: a multivibrator of which the ON time is 12 milliseconds while the OFF time, in the free-running condition, is about 0.85 second. Accordingly, the multivibrator employs two transistors of opposite conductivity types. During the brief ON time, both transistors conduct. During the much longer OFF time, neither conducts. In addition to combining the functions of two components of the earlier apparatus, this approach conserves battery power and prolongs battery life.

In the earlier apparatus the simulated refractory delay time was provided by the OFF time of one transistor and the ON time of the other. With the present apparatus it is provided by a reverse bias on a diode, initiated by the outgoing pulse. This bias gradually decays during the interpulse interval. With proper selection of the magnitudes of circuit elements this reverse bias falls, in the required refractory time of about 0.3 seconds, to a level at which it is no greater than the amplitude of a pulse from the AV delay simulator whereupon, though not earlier, this pulse is passed by the back-biased diode to the multivibrator, thus to cause generation of a new heart-stimulating pulse.

The invention will be fully comprehended from the following detailed description of an illustrative embodiment thereof, taken in connection with the appended drawing of which the single figure is a schematic diagram of apparatus embodying the invention.

Referring now to the drawing a signal, known as a P-Wave, which may be picked up by an electrode implanted in, on, or near the atrium of the heart passes through a switch S, when the latter is in position B, through a filter resistor R and capacitors C C to the input point of a two-stage amplifier of conventional construction including transistors Q and Q with conventional interconnections among them. Power and bias are supplied to this amplifier and to other component parts of the apparatus, by sections B 8 of a battery. Advantageously, each battery 'section is a mercury cell, and each one contributes about 14 volts. A Zener diode Z1 having a breakdown potential of about 3 volts is included to protect the circuit against spurious voltages of excessive magnitudes. The filter excludes components of about 10 c.p.s. and lower and of about 200 c.p.s. and higher. Moreover it introduces a slight delay.

This signal, after being slightly delayed by the filter.

and brought to a suitable level by the amplifier, is passed through a blocking capacitor C to the base electrodes of transistors Q and Q interconnected to operate as a monostable multivibrator serving as an AV delay simulator. The collector-base cross coupling paths are such that, in the stable state, Q conducts while Q is cut off and in the metastable state the conduction is reversed between the two. The incoming signal from the amplifier is applied to the base of Q through a diode D and to the base of the transistor Q thorugh a diode D These two diodes are oppositely poled so that the incoming amplified signal shall switch the monostable multivibrator from its stable state to its metastable state independently of the polarity of the signal. The anode of D and the cathode of D are returned through R to a suitable point of a voltage divider constituted of R and R connected across battery sections B and B This ensures freedom for the signal applied to the diodes to swing either positively or negatively with equal sensitivity. Thus a positive signal is applied through the diode D to the base of transistor Q while a negative signal is similarly applied through the diode D to the base of transistor Q The action, in either case, is to shift the monostable multi vibrator to its metastable state. This construction relieves the surgeon of the need to implant the atrial pickup electrode in a particular part of the heart at which the wave to be sensed is known to be of preassigned polarity.

Each time the monostable multivibrator is thus shifted from its stable state to its metastable state it delivers a positive pulse a, and hence a positive spike signal b through a capacitor C Were it not for the diode D this signal would be delivered to the free-running multivibrator constituted of transistors Q and Q But because of the fashion in which this diode is connected and poled, this positive signal is blocked by the diode D and is thus ineffective to alter the state of the free-running multivibrator. But when, after the lapse of the built-in AV delay, of the order of 0.12 second, the monostable multivibrator (Q -Q returns from its metastable state to its stable state, a negative spike signal 0 of like magniude is passed through the capacitor C to the free-running multivibrator (Q Q With this polarity the signal passes through the diode D if not reversely biased as described below, and is applied to the base of transistor Q causing it to conduct.

In contrast with the apparatus of the specification referred to above, the present free-running multivibrator employs complementary transistors; i.e., transistors of opposite conductivity types. Hence, when the transistor Q commences to conduct then, through regenerative action,

the transistor Q commences to conduct too, through a diode D This diode, poled as shown, is included merely to protect the transistor Q,- from reverse voltages. Conduction of these two transistors endures for a pre-set time, determined by the magnitudes of C and R illustratively about two milliseconds. This period is appropriately termed the CN time of the multivibrator. The multivibrator then returns by itself to its OFF state in which both of the transistors remain non-conductive for the duration of the interpulse interval, more or less, depending on whether, in the absence of a signal through the capacitor C it runs free at the pre-set rate of about 70 beats per minute, which means an interpulse interval of about 0.85 second, or whether its switching is advanced on the time scale by a signal picked up from the heart, amplified, delayed by the AV delay simulator, and applied to the multivibrator Q -Q through the diode D thus to shorten a particular interpulse interval. The duration of the intrinsic (not externally influenced) interpulse interval is determined by the magnitudes of C and of R 5,

R R and R of which R is the dominant one. Any of these may be variable, thus to alter the intrinsic interpulse interval. Thus, with each output pulse delivered by the multivibrator Q -Q whether intrinsic or induced, a new interpulse interval is initiated.

In the case of a pacer that is totally implanted within a patients body, economy of battery power is of high importance. The co-pending specification referred to above includes an externally operated battery charger, but this adds circuit complexity. In contrast, the present system offers a substantial economy in battery power and does so by employing, for the free-running multivibrator, transistors of opposite conductivity types. With this arrangement, when the ON time is of the order of one or two milliseconds while the OFF time is of the order of .85 second, the total conduction in the course of each pulse cycle is very much reduced as compared with the total conduction in the prior arrangement with which either one transistor or the other conducts all the time.

The output pulse of the free-running multivibrator (Q Q appears as a voltage drop across the resistors R and R A tap 2 picks up a preassigned fraction of this voltage and delivers it to the baSe electrode of a power transistor Q which then conducts, permitting current to flow from the positive terminal of the battery stack, through a bias resistor R to the negative battery terminal. The voltage appearing across the bias-resistor R is passed by way of a blocking capacitor C to an electrode 3 im-bedded in or near the ventricle of the heart. As in the foregoing specification the electrical circuit is completed through another electrode 4 located in the patients abdomen or chest cavity. This may advantageously be a metal shield which surrounds the implanted apparatus. The resistors R and R are preferably of a magnitude to provide a constant current output.

A capacitor C interconnects the tap 2 with the negative battery terminal. It serves merely to bypass small, accidental transient voltages and so to prevent their operating the power transistor Q A Zener diode Z having a breakdown potential of about 8 volts, connected between the abdominal electrode 4 and the ventricular electrode 3, serves to protect the circuit against damage which might occur, for example, due to electrical stimulation of some part of the patients body either by a stray field or as a consequence of some unrelated medical procedure.

It is necessary in any electronic pacer which responds to atrial or ventricular signals to provide electrical simulation of the refractory period of the heart. In the specification referred to above this was provided by the ON time of the free-running multivibrator. In the present case, the ON time of the multivibrator constructed of transistors Q and Q endures for about two milliseconds i.e., very much less than the required refractory period. It thus becomes necessary to make other provision for simulation of the refractory period.

In the present apparatus this is provided by the combination of resistors R R R R R capacitors C and C and the diodes D and D This subcircuit operates in the following fashion. During the brief conduction time of transistors Q and Q, the capacitor C is subjected to nearly the entire battery voltage and hence acquires a charge of nearly 7 volts. Charging of C takes place with great rapidity through Q D and D now forwardly biased. During the OFF time of the two transistors, this charge gradually leaks away through R and R (The charge on C leaks away much more rapidly and its influence on the simulated refractory time can be disregarded.) The cathode of the diode D is connected through R to the positive terminal of C Therefore, the diode D is biased in the reverse direction so that it is substantially non-conductive, even to a negative pulse which may reach it through the capacitor C from the AV delay simulator. As the charge leaks off the capacitor C the back bias of the diode D gradually falls. In the course of its fall it eventually reaches a point at which its back bias is no greater in magnitude than a negative spike signal 0 which might reach it through the capacitor C At this point of the discharge cycle, therefore, and not before, the diode D; can pass such a negative signal to the base of Q causing a shift of the state of the multivibrator, Q Q both transistors Q and Q becoming conductive, thus advancing the next multivibrator output pulse on the time scale. By appropriate co-ordination of the magnitudes of the various resistors of the circuit, any of which may be variable, and of the capacitances of the capacitors C and C the simulated refractory period thus provided ma readily be adjusted to a desired duration, for example, 0.325 second.

A patient requires an artificial heat pacer, electronic or otherwise, only when the physiological behavior of his heart is, at best, doubtful. Hence, it must always be contemplated that the P-Wave recovered by the atrial electrode may at any time fail or become unreliable in which case the apparatus described in the specification referred to above can operate only at its pre-set rate, for example, beats per minute or at undesirable P-Wave rates. But though the atrial signal fails the ventricle may still be in condition to deliver a spontaneous sensible QRS complex which may, indeed, cause a contraction of the ventricle in the normal physiologic fashion. Under these conditions, with the apparatus of the foregoing specification, the ventricle may undergo contractions under the influence of two unrelated stimuli; first, its own physiologic contractions induced by its own QRS complexes and second, the stimulated contractions induced by the pacer apparatus, now running at its pre-set rate. The result may be a very troublesome cardiac arrythmia.

In the present invention this difliculty is prevented by switching the input point of the amplifier from the atrial electrode 1 to the ventricular electrode 3, that is to say by throwing the switch S from position B to position A. With the switch in position A, any QRS complex picked up by the ventricular electrode 3 and following the last sensed ventricular potential, QRS complex or stimulus, by more than the simulated refractory period is amplified, passed through the AV delay simulator, and applied to the free-running multivibrator Q Q This causes the multivibrator immediately to switch from its OFF state to its ON state. If, to the contrary, the most recent potential sensed by the ventricular electrode 3 occurred within the simulated refractory period the pulse from the AV delay simulator is blocked by the diode D When not so blocked and the free-running multivibrator Q -Q is thus switched, its output pulse is delivered as a stimulus to the ventricle electrode 3 in the fashion described above. Because the delay interposed by the AV delay simulator is of the order of A second, while the refractory period of the heart itself, following its last preceding contraction, is of the order of one-quarter second, the stimulus thus delivered by the multivibrator in response to a natural sensed QRS complex reaches the heart during the hearts own physiological refractory period and is thus ineffective to initiate a contraction. It has been found, furthermore, that delivery of such artificial stimuli to the heart during its refractor period, even large numbers of such pulses in sequence, are not known to be physiologically harmful. To the contrary, provision for delivery of such stimuli to the heart provides valuable insurance that the required contraction of the venticle shall take place either under the influence of its own spontaneous QRS complex through physiological means or, if natural transmission should fail, through the influence of the pacer. This is because, in the absence of a spontaneous QRS complex, the multivibrator, now switching at its pre-set rate, i.e., running free, delivers its stimuli at times when the heart is not in a refractory condition and can readily respond to them. At the same time, the artificial refractory delay simulator constituted of the diode D the capacitor C and associated resistors ensures that no two such events shall take place at intervals of less than about 0.4 second of each other.

When the ventricular electrode 3 is performing its sensing function it cannot, of course, distinguish between a heart potential delivered as a stimulus by the pacer and one which originates physiologically in the heart itself; i.e., a QRS complex. Hence each pacer stimulus is picked up "by this electrode 3 and, when the switch is in position A, amplified, and applied to the monostable multivibrator (Q -Q If, and only if, the latter is in its stable state, a pulse is passed on, after the termination of the simulated AV delay, to the diode D If, and only if, the latter is in its non-refractory state, this pulse trips the free-running multivibrator Q Q causing it to generate a new pulse and deliver it as a stimulus to the heart. If, when the heart potential is sensed, the monostable multivibrator (Q Q is in its metastable state,

the sensed heart potential is blocked at this point of the circuit, If it passes this point, it may still be blocked by the diode D if in its refractory state. This arrangement ensures that, even though extraneous interfering influences may be sensed, either by the atrial electrode 1 or by the ventricular electrode 3, no two consecutive stimuli can be delivered to the heart unless they are spaced apart on the time scale by 0.4 second.

When the entire pacer apparatus is implanted in the patients body, manual operation of the switch S is impossible without surgery. Instead, a reed switch may be employed having a small margin of mechanical stability in each of two different positions. This stability may be imparted by a suitable bias, e.g., a magnetic bias. The switch S may then be thrown from either position to the other by the field of an external magnet which, when the magnet is brought close to the pacer, threads the body wall and tissues and actuates the switch, If preferred, a thermally-controlled switch may be employed, or one that responds to radio frequency or video frequency pulses, or even to a mechanical shock.

Similar techniques may be employed, if desired, to alter or adjust the magnitudes of resistors which control the spontaneous pulse rate of the free-running multivibrator and the magnitude of its output current.

The invention having now been described, what is claimed is:

1. A cardiac pacer comprising a source adapted to generate electric pulses at a preassigned repetition rate conformable to the rate of the normal heart beat of a patient, means including a ventricular electrode for applying said pulses as stimuli to said heart, means for sensing a heart potential, said pulse source comprising a freerunning transistor multivibrator and an R-C cross coupling path extending from the collector of each transistor is the base of the other transistor whereby said source, absent an incoming physiologic potential, generates a periodic sequence of pulses separated by interpulse intervals, and means under control of said heart potential for advancing the next source pulse on the time scale, thus to initiate a new source period, said advancing means including a diode disposed to receive an incoming signal, and, when not reversely biased, to pass said signal to the base of one of said transistors to advance the next free running pulse due from said multivibrator source on the time scale, a supplementary biasing capacitor, connections for charging said supplementary capacitor to a preassigned potential on the occurence of each pulse of said source, connections through which said capacitor is gradually discharged during the interpulse interval, thus to cause gradual decay of the potential of said capacitor, and connections for applying said decaying potential as a reverse bias to said diode, whereby said diode acts to block a signal from reaching said pulse source until after said bias potential has decayed to a magnitude no greater than the magnitude of said signal, said diode, supplementary capacitor, charging connections and discharging connections together constituting a refractory delay simulator.

2. Apparatus as defined in claim 1 wherein said sensing an atrial electrode for sensing an atrial heart potential and said ventricular electrode for sensing a ventricular heart potential, and further comprising switching means for selecting either said atrial electrode or said venricular electrode to sense said heart potential.

References Cited UNITED STATES PATENTS 3,311,111 3/1967 Bowers 128-422 3,345,990 10/ 1967 Berkovits 1284 19 FOREIGN PATENTS 826,766 1/ 1960 Great Britain.

OTHER REFERENCES Nathan et al.: Progress in Cardiovascular Diseases, vol. 6, No. 6, May 1964, pp. 538-565 (only p. 542 relied on).

WILLIAM E. KAMM, Primary Examiner.

U.S. Cl. X.R. 33ll13 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,433,228 March 18, 1969 John Walter Keller, Jr.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 21, "refrectory should read refractory Column 4, line 16, "thorugh" should read through Column 6, line 15, "heat should read heart line 65, "venticle" should read ventricle Column 8, line 3, "is" should read to line 28, after "sensing" insert means includes line 32, "venricular should read ventricular Signed and sealed this 31st day of March 1970.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer 

